Active matrix/organic light-emitting diode (AMOLED) display device is one of the research hotpots for a current flat-panel display device. Comparing with liquid crystal display devices, the OLED has such advantages as low power consumption, low production cost, self-luminescence, wide viewing angle and rapid response. Currently, OLED has begun to replace the typical LCD display panel in mobile phones, PDA, digital cameras and other display products. The design of a pixel driving circuit is a core of the AMOLED display device, and thus it is of important research significance.
Different from a TFT-LCD which controls the brightness with a stable voltage, the OLED is driven to emit light with a stable current. As shown in FIG. 1, an original 2T1C-based pixel circuit of the AMOLED display device consists of a driving thin film transistor (TFT), a switching TFT and a storage capacitor Cs. When a certain row of pixels is scanned by a scanning line, Vscan is at a low level, the switching TFT T1 is turned on, and a data voltage Vdata is written into the storage capacitor Cs. When the scanning is ended, Vscan is changed to be at a high level, T1 is turned off, and the driving TFT T2 is driven by a gate voltage stored in Cs so as to generate a current for driving the OLED, thereby to ensure the OLED to emit light continuously within one frame. A saturated current IOLED of the driving TFT may be calculated by the equation IOLED=K(VGS−Vth)2.
Due to a manufacturing process and the aging of elements, a threshold voltage (Vth) of the driving TFT for each pixel point will be drifted, which results in a change of the current passing through the OLED for each pixel point along with a change of the threshold voltage. As a result, the display brightness is uneven, and thereby a display effect of an entire image will be adversely affected.